The following description sets forth the inventor's knowledge of related art and problems therein and should not be construed as an admission of knowledge in the prior art.
A resin foamed sheet having a heat shrinkable nature, such as, e.g., a foamed polystyrene resin foamed sheet, has been widely applied to various products, such as, e.g., cups, plates, food containers, packaging materials, or packaging labels, in various fields. Particularly, in the case of a heat shrinkable foamed sheet to be used as packaging labels, decoration printing is performed on a surface of the label applied to a glass/plastic container to provide its content's information.
This kind of heat shrinkable resin foamed sheet can be manufactured by a method disclosed in, for example, U.S. Pat. No. 5,925,450 (JP, H07 (1995)-64005, B), U.S. Pat. No. 3,979,000 (JP, S61 (1986)-21826, B), U.S. Pat. No. 5,342,560 (JP, H05 (1993)-505774, A), and JP, H07 (1995)-32525, A.
For example, a general-purpose polystyrene resin as a raw material of a foamed sheet, a nucleus forming agent for forming foamed cells, a spreading agent, and white mineral oil or polybutene acting as a viscosity controlling agent are introduced into a first extruder to be melted therein while adding polymeric ester such as ditridecyl adipate (DTDA) as a plasticizer. Nitrogen or carbon dioxide acting as a foaming agent is added under high pressure to the molten polystyrene and kneaded, and then the resulting melt is transferred to a cooling extruder provided at a rear part of the first extruder to be cooled therein. The transferred melt is cooled to a temperature appropriate to the extrusion, and then sent to an annular die portion provided at the front end of the cooling extruder. On the other hand, non-foamed polystyrene resin is introduced into a second extruder and melted therein, and then transferred to the die portion provided at the front end of the cooling first extruder in a state in which no foam is contained in the molten resin. In this die portion, the extrusion material from the first extruder and the extrusion material from the second extruder are joined together and co-extruded as a cylindrical two-layered sheet in which a non-foamed resin layer is coated on a foamed resin layer.
The inner and outer surfaces of the cylindrical sheet co-extruded from the extrusion die are cooled with air and/or a cooling mandrel. At this time, the sheet is stretched by blowing between the extrusion die and the mandrel ring having a large diameter at a temperature range higher than a softening point of the resin, and then cooled to a temperature which is the softening point or below thereof on a cylindrical mandrel provided at the downstream side of the mandrel ring. During the cooling, tension is applied using nip rolls to be stretched in a longitudinal direction of the sheet, and the stretched sheet is cut in halves with a cutter. The resulting sheet having two layers, i.e., a foamed layer and a non-foamed layer, will be wound.
Especially, in the foamed sheet of this kind to be used as a packaging material, such as, e.g., a label, in addition to mechanical characteristics, such as, e.g., tensile strength, rigidity, shear force, flexibility, and heat shrinkable characteristic, the printing characteristic is also one of important characteristics to provide decoration for giving a visual product image on the surface of the sheet.
In a conventional heat shrinkable foamed sheet, however, such print images with clear print boundaries could not be obtained, for example. Thus, the printing performance was not always satisfactory.
Under the circumstances, the inventors performed various experiments and studies to provide a heat shrinkable foamed sheet excellent in surface glossiness, surface smoothness and printing nature. As a result, they found the fact that in a multi-layer foamed sheet having at least a non-foamed layer as a skin layer small in thickness and a foamed layer as a core layer larger in thickness than the non-foamed layer, various dimensions, such as, e.g., surface smoothness of the non-foamed layer and sizes of foam cells formed in the foamed layer, exert a strong influence on a printing performance of the non-foamed layer constituting the surface skin layer. The inventors further continued experiments and studies and finally made the present invention.
The description herein of advantages and disadvantages of various features, embodiments, methods, and apparatus disclosed in other publications is in no way intended to limit the present invention. Indeed, certain features of the invention may be capable of overcoming certain disadvantages, while still retaining some or all of the features, embodiments, methods, and apparatus disclosed therein.